This invention relates to removal of slag from molten metal, and more particularly, it relates to a method and apparatus highly efficient in the removal of slag from the surface of a body of molten metal.
In the prior art it is known to remove slag from the surface of molten metal by raking. However, this approach is not without its problems. That is, when a rake is used to pull slag from the surface of a body of molten metal, the operation does not discriminate, and often a large amount of molten metal is removed with the slag which interferes with the efficiency of the process. The raking approach is inefficient in yet another way. That is, the raking operation can leave as much as 15 to 25% of the slag behind and thus is not very efficient in making a separation of slag from molten metal. Thus, there can occur a high level of undesirable inclusion in cast metal resulting from this practice. It is important to reduce the level of slag on the surface of the molten metal to avoid poring metal therethrough or therewith which contributes to dirty steel.
U.S. Pat. No. 3,761,242 discloses the use of a porous plug mounted in the bottom of a crucible near or adjacent a tap hole, and purging gas is introduced through a porous plug. According to the patent, the purging gas speeds the refining process and pushes the slag away from the tap hole or discharge spout to permit pouring of molten metal. However, in this operation, the slag still remains in the crucible and often it is difficult to remove all the molten metals without removing slag. Alternatively, to avoid removing slag, some molten metal is often left in the crucible. This approach has another disadvantage or problem. That is, the porous plug involves a safety issue, if the porous plug becomes dislodged, and molten metal would draw uncontrollably from the crucible. Further, another safety problem arises when the plug has to be replaced. Thus, the plug can result in a weak point in the crucible.
U.S. Pat. No. 4,517,015 discloses the introduction of gas through a porous plug in the bottom of the crucible and the introduction of gas through a lance projecting through a layer of slag. According to the patent, this produces a duplex stirring effect that produces stirring in a dead zone D, and this shortens the time length required for stirring during the suspension of the electric power supply.
From the above, it will be seen that there is a great need for a method and apparatus that permits the removal of slag from molten metal in a highly efficient and safe manner. The present invention provides such a method and apparatus.